Dinas Ffareon is an Iron Age hill fort near Beddgelert which overlooks Lyn Dinas in Snowdonia. It is one of the more remote castles in Wales and “it was here that King Lludd ab Beli buried the two dragons which fought each other, as told in the Welsh epic the Mabinogion.”

Later tales (Nennius’ and Geoffrey of Monmouth’s among them) tell of King Vortigern retreating back into Snowdonia and choosing Dinas Ffareon as the place to build his fort.

Unfortunately for him, each night the ground was shaken such that the fort fell down. The King’s advisors stated that a fartherless child had to be sacrificed in order to stop the fort tumbling. Myrddyn Emrys (Merlin) and Emrys Wledig (Ambrosius Aurelianus) come into the story as well.

“Merlin prophecised that the Red Dragon represented the Britons and the White Dragon the Saxons and that the event meant that the Britons would be victorious over the Saxons. The Celts tended to refer to leaders as dragons (draig) so one could also read it as meaning the leader of the Britons being victorious over the leader of the Saxons, something which came to pass through Uther Pendragon and then Arthur himself.”

http://www.wyrm.org.uk/ukdracs/dinasemrys.html

http://www.caerleon.net/history/geoffrey/Prophecy1.htm

Dinas Ffareon, now Dinas Emrys (renamed, of course, for Merlin), sits atop a rock that is one of the strongest, natural fortifications in Wales.  The remains of the medieval stone fort, possibly built in the 13th century by either Llywelyn Fawr or Llywelyn ap Gruffydd, still top it.  Underneath, stones date to the Iron Age.

Modern archaeology reveals: “Dinas Emrys was occupied to some extent in the late Roman period, but that rough stone banks around its Western end are later. They were poorly built of stone two or three times and took strategic advantage of natural crags. Still less substantial walls were also discovered to the north and south. Broken sherds of Eastern Mediterranean amphorae, Phoenician red slip dishes and a pottery lamp roundel featuring a Chi-Rho symbol indicate that these features do indeed date to the 5th and 6th century.”
http://www.earlybritishkingdoms.com/archaeology/emrys.html
http://www.castlewales.com/dinas_em.html